1. Field of the Invention
The present invention relates to a brushless motor driving apparatus and a brushless motor driving method.
2. Description of Related Art
As a technique for driving a brushless motor at low speed without a sensor, one in which energization mode is switched according to induced voltage (magnetic saturation voltage) in a non-energized phase has been proposed, as described in JP 2009-189176 A.
However, in a brushless motor drive technique as described above, there is a possibility that the duty ratio for a PWM (Pulse Width Modulation) control increases, thereby causing synchronism loss in the brushless motor. That is, in a state in which the brushless motor is feedback-controlled in an extremely low speed area, when a brushless motor does not reach a target rotational speed due to, for example, a high load, control is performed to increase the duty ratio. In this case, electric current also increases with the increase in the duty ratio of the brushless motor. Accordingly, it is necessary to limit the electric current.
There are two types of electric current; one is phase current, and the other is power source electric current. When a load is high, the phase current is limited earlier than the other. In a state in which the phase current is limited, rotational speed of the brushless motor gradually increases or coil resistance increases due to a coil temperature increase, so that the duty ratio or power source electric current increases with respect to the same phase electric current. As a result, a rate of change in magnetic flux between the energized phases decreases, and hence an induced voltage decreases. Consequently, energization modes for the brushless motor fail to switch with appropriate timing so that synchronism loss is caused in the brushless motor.